Parallel batteries

[Sam]

Any battery experts here? Help me understand this senario:

Two separate, working 12 volt lead-acid batteries:

#1) Smaller, 60AH capacity, rest voltage = 12.5V (about 70% charged)
#2) Larger, 120AH capacity, rest voltage = 12.5V (about 70% charged)

Now connect the two of them together in parallel. Leave them connected for an hour or so. Disconnect, and let the batteries rest for 30 mins. Measure voltage.

I would expect the same voltage, 12.5, for both batteries. In other words, there was no current flow in either direction while the batteries were connected, because the voltage were the same. The batteries' capacities did not matter when connected like this.

Am I wrong?

Thanks,
Sam

 

[SC]

Yes, if they are at 12.5V at the start, they should remain at 12.5V. I'm assuming by "in parallel" you are connecting + to +, and - to -.

There will be a small current flow to equalize the voltages as they will probably internally self-discharge at different rates, but I don't see anything wrong with your reasoning.

Was that not what you observed?

 

[CyclingInvestor]

Also, there will be very minor current flow due to unequal heating / cooling rates during the day (different size batteries), which changes chemical reaction speeds.

 

[ls99]

What they said. The point of the exercise is?

Normally if expecting to use them as power source connected in parallel, it is preferable to have equal capacity, equal age batteries.

 

[Sam]

Thanks, SC and CyclingInvestor.

What they said. The point of the exercise is?

Normally if expecting to use them as power source connected in parallel, it is preferable to have equal capacity, equal age batteries.

I do plan to use them together as a single 12V power source. But I keep hearing recommendations like yours, and I don't understand why.

The way I see it, when a load is applied, each battery will provide a current proportional to its capacity, 1/3 from #1 and 2/3 from #2. The reverse is true under charge. There will be no (or only negligible) current going from one battery to the other. So, I don't see any problem.

What am I missing?

Sam

 

[FIRE'd@51]

I'm no battery expert, but it seems to me that when you start drawing currents, the internal resistances of each battery will cause the output voltages to be unequal, so that when they finally equilibrate, the resultant output volatge of the parallel combination will be less than 12, and this output voltage would fluctuate with the amount of current drawn.

For example, if the internal resistance of each battery were the same and each was delivering its maximum current, the voltage drop across this internal resistance, would be twice as much in one battery as the other, so one battery will be continously charging the other. Presumably, if the two batteries are identical, the voltage drops across each internal resistance will be the same.

 

[ERD50]

Sam, I'd agree that it is *preferable* to have equal capacity, age, type of batteries.

It is not as simple as just the AH ratings, there are curves associated with all these parameters, and they can vary by current draw, temperature, time, etc.

However, things will probably work generally as you say, as one battery's capacity is drawn down, the other will fill in until they are both depleted (don't discharge below 10.2V, that can destroy a lead-acid battery quickly).

What is your application? What currents and times do you except to see?

-ERD50

 

[ERD50]

... the voltage drop across this internal resistance, would be twice as much in one battery as the other, so one battery will be continously charging the other.

Sort of. I think you will find that the lower AH rated battery will have a higher internal resistance. That is usually a function of the capacity (but there can be other variables at play).

The more likely scenario is that the R of one is greater than the other. The lower R (higher capacity battery) will provide the higher current of the two, which is just what you want. They should equalize - to a degree.

As long as you are drawing current out of the batteries, one will not be charging the other. That could happen if you have drawn them down, and then remove the load. At that point, one might float up to a higher rest voltage than the other, and charge it for a while - then they will equalize again.

I don't have any real-life experience with lead-acid in parallel, but I'm pretty sure they just bolt 'em together. I don't think you need to be too worried, but as pointed out, equal size/type will be optimum.

-ERD50

 

[SC]

The way I see it, when a load is applied, each battery will provide a current proportional to its capacity, 1/3 from #1 and 2/3 from #2. The reverse is true under charge. There will be no (or only negligible) current going from one battery to the other. So, I don't see any problem.

I have not measured this personally, but what I would expect is that each battery would provide a current in proportion to the different resistances between the batteries and the load; some of these resistances would be internal to the battery, and some external (e.g. battery terminal, wires). Then as the voltage between the batteries starts to differ, the discharge rate will differ between the two batteries to equalize the voltage so that in practice, over time, you would end up with the current approximately proportional to the capacities like you expect.

The above is just speculation on my part, as it's been a long time since I did bench measurements of actual batteries. I know batteries can have very different graphs of voltage vs. capacity, so there may be times when you are drawing almost exclusively from one battery or the other. As long as this is okay, and you are using two batteries for extended life rather than high current, I don't see any issue with what you're doing.

I wonder if the recommendation to use batteries of similar capacity, type, and age has more to do with the charging side of the equation. If you are not planning to remove these batteries to charge them individually, then I would worry more about having the same situation apply during charging, where one battery temporarily receives most (maybe all) of the charging current. If the charging is slow enough, this shouldn't be a problem. I would also worry about the batteries having slightly different "fully charged" voltages and the possibility of overcharging one of them. I can't think of a good solution for that off the top of my head, other than to match the batteries. Again, speculation on my part -- I would be curious to hear from anyone with experience in this matter.

EDIT: I missed ERD50's post before replying. I think he said it better but we're on the same page. I tweaked my first paragraph a tiny bit to clarify.
EDIT2: Tweaked again. I'll stop now.

 

[ls99]

Fire'd@51's discussion is pretty good.

Plus the smaller capacity battery will be in deep discharge condition a good while longer than the larger capacity battery while current is being drawn.

Excessive discharge depth causes sulfation. Resulting in short battery life. A sulfated battery will not accept charge, until sulfation is sufficient to bend the plates and short them out. Once it has a shorted cell, the battery is junk.

Happey experimenting.

 

[Sam]

Thank you all. I'm beginning to see the nuances. I know I could count on you guys!

I have not measured this personally,

Neither have I. I was speculating on that. I did do the experiment, but did not have an DC amp meter handy (I'm building one). A 90W load was put on for about 5 mins on the combined battery. The parallel connection was disconnected, and the batteries were allowed to rest for 15mins. The voltage of both batteries were the same, 12.4V.

EDIT2: Tweaked again. I'll stop now.

I don't mind, keep on tweaking

What is your application? What currents and times do you except to see?

I just acquired a cheap old toy in December. It's a small 22' class C motorhome. Since then, I have put in over 4,000 miles, and have been on the road for about 30 days/nights. It has a 2 years old house 60AH marine battery (separate and isolated from the engine battery). My daily consumption is about 40AH, which is not good for the battery. Sometimes, I draw it down to 10V (I know, it's bad). Low enough to cause the 30W neon light to dim and flicker, and the propane fridge to complain.

I know I need to upgrade my battery, but don't like the idea of throwing away a good working battery, thus my dilemma.

Sam

 

[SC]

Your RV tour sounds like quite an adventure!

The old battery may not be worth saving, especially if its been down to 10V a few times. Most (all?) battery shops will take your old battery for recycling.

I have heard some RVers will buy 6V golf cart batteries and make a bank of 2, 4, or even 6 to get more capacity. I don't know what they cost compared to marine batteries but that could be an option. Our trailer came with a car battery installed, which is next to useless in that application, but I guess it saved them a few bucks.

Good luck!

 

[REWahoo]

Sam,

Here is a great source of 12v system and battery information for those of us who venture into the RV world:

The 12volt Side of Life (Part 1)

 

[Sam]

Your RV tour sounds like quite an adventure!

Thanks, it was a lot fun. This is my first motorhome. It took a while for me to realize the benefits of having one. 3 things were on my minds when touring/traveling before: Where to stay, what to eat, how to get there? The small motorhome solves all three!

I have heard some RVers will buy 6V golf cart batteries and make a bank of 2, 4, or even 6 to get more capacity.

I know. Some of their battery banks are humogous, and expensive!
Deep Cycle Battery

Sam,

Here is a great source of 12v system and battery information for those of us who venture into the RV world:

The 12volt Side of Life (Part 1)

Thanks, REWahoo. I've gone through that and several other online docs. The info was generally good. But sometimes [-]misleading[/-] ambiguous. For example: On one hand they promote deep cycle batteries because these are designed for deep discharge up to 80%. On the other, they said to never go below 50% for longevity!

Sam

 

[REWahoo]

The info was generally good. But sometimes misleading ambiguous. For example: On one hand they promote deep cycle batteries because these are designed for deep discharge up to 80%. On the other, they said to never go below 50% for longevity!

You mean these statements?

Deep cycle batteries are designed to be discharged down as much as 80% repeatedly...

Battery life is directly related to how deep the battery is cycled each time. If a battery is discharged to only 50% each cycle, it will last about twice as long as if it is cycled to 20%...

No different than my car. It's designed to go 120 MPH but I'm sure it will last much longer if I don't drive it faster than 70.

If you have room, I recommend two 6v golf cart batteries wired in series to produce 12v.

 

[Sam]

No different than my car. It's designed to go 120 MPH but I'm sure it will last much longer if I don't drive it faster than 70.

Duh. Makes sense. I should have looked at it that way.

If you have room, I recommend two 6v golf cart batteries wired in series to produce 12v.

I do have room, and I like the T-105 too (actually, I don't know anything about it, but everyone else talks about it, so it can't be bad.) But it's too costly. $1.11/AH as opposed to $0.64/AH for Walmart Everstart Maxx marine battery. I think I will go with Everstart battery and not [-]driving it faster than 70[/-] discharging it below 50%.

Sam

 

[REWahoo]

I do have room, and I like the T-105 too (actually, I don't know anything about it, but everyone else talks about it, so it can't be bad.) But it's too costly. $1.11/AH as opposed to $0.64/AH for Walmart Everstart Maxx marine battery. I think I will go with Everstart battery and not [-]driving it faster than 70[/-] discharging it below 50%.

Sam

Be sure to factor in longevity. I never got more than 2 years out of the 12 battery on my travel trailer but the two T-105's I have now are still going after 5 years....even though they've been driven well over 70 several times.

 

[nphx]

Would diodes off each battery somewhat isolate the draw loads? Also diodes on the charge circuit?

 

[Sam]

Would diodes off each battery somewhat isolate the draw loads? Also diodes on the charge circuit?

I think it would.

But they add a level of complexity to a very simple system. Not to mention labor cost (although my time does not worth much, and I would be tinkering anyway). I'll just go with a pair of monozygotic batteries, and hope they will behave similarly as they age.

Sam

 

[ERD50]

I don't see where diodes would help at all, only hurt. True, it would isolate the batteries, but what would that accomplish? The load is still connected to both, so it will still draw down both just as it did before. You would just have diodes in the path, wasting power and doing nothing else.

The only thing the diodes would do is keep one battery from charging the other when no load is drawn, but I don't think that is a real issue - they have to equalize anyhow.


The cons are you will lose 0.3V (if you can find high current, low Vf diodes), so thats 3% loss right there. And, you would need two chargers.

What about those Optima batteries? My brother says they are great, and not so much more expensive these days.

-ERD50

 

[Sam]

An update:

I bought 3 new identical group 27 batteries, ~100AH each. Fully charged all 3 separately. Combined them into one 300AH battery. Installed 4 precision shunts (100A/100mv): One for each battery, and one for the combined battery. Start drawing a 15A load from the combined battery and watch the current going out:

Combined current = 15A
Battery #1 = 5.5A
#2 = 4.5A
#3 = 5.0A

So, even identical (same brand, same type, same capacity, same month of manufacturing) batteries do not behave identically. Same observation when the batteries are under charge.

Sam

 

[landonew]

An update:

I bought 3 new identical group 27 batteries, ~100AH each. Fully charged all 3 separately. Combined them into one 300AH battery. Installed 4 precision shunts (100A/100mv): One for each battery, and one for the combined battery. Start drawing a 15A load from the combined battery and watch the current going out:

Combined current = 15A
Battery #1 = 5.5A
#2 = 4.5A
#3 = 5.0A

So, even identical (same brand, same type, same capacity, same month of manufacturing) batteries do not behave identically. Same observation when the batteries are under charge.

Sam

Are you asking why this is?

My theory would be that the batteries are designed to operate within certain parameters. I would guess that the internal resistances are slightly different. Remember, a DC battery is essentially a capacitor. Ideally, the capacitor would have an infinite resistance. However, practically the capacitor is not a perfect inductor.

To expound on the automobile example, Imagine you have three trucks pulling three trailers down a 3 lane highway, Each going from point A to point B.

You set the cruise control of each truck at 70 mph. You wouldn't expect each truck to be going EXACTLY the same speed. You may have one truck going 70.00015, while another is going 69.99985 mph.


From an engineering perspective, the method for controlling the outputs of each source would be to add an appropriately sized resistor in series with each individual battery prior to your junction point. However, I wouldn't recommend that for your application.

 

[ERD50]

An update:

I bought 3 new identical group 27 batteries, ~100AH each. Fully charged all 3 separately. Combined them into one 300AH battery. Installed 4 precision shunts (100A/100mv): One for each battery, and one for the combined battery. Start drawing a 15A load from the combined battery and watch the current going out:

Combined current = 15A
Battery #1 = 5.5A
#2 = 4.5A
#3 = 5.0A

So, even identical (same brand, same type, same capacity, same month of manufacturing) batteries do not behave identically. Same observation when the batteries are under charge.

Sam

Yes, there will always be some variation battery to battery, and this is what you will see in parallel. However, if they are very similar, I bet this evens out over a longer set of charge/discharge cycles. You added another variable by charging separate, then discharging in parallel.

As landonew mentions, trying to do something about it will cause more problems than it solves. You might just have to accept that 3 batteries will give you maybe 2.9x the amp hours rather than the theoretical 3x.

-ERD50

 

[Sam]

You added another variable by charging separate, then discharging in parallel.
-ERD50

I only charged them separately once, after bringing them home from the store. I wanted to make sure they are all fully charged before combining them. Now, they are all charged together as one. I have gone through several charge/discharge cycles and the individual currents through each battery still differs.

Sam

 

[ls99]

The small current differences you found are not significant. If you used different capacity or age batteries the differences would be much larger, especially after several hours of use.
Having 3 100AH batteries is an excellent choice. You are keeping the discharge rate around (C=capacity) C/20, which will provide good longevity. Providing they don't get deep discharged.
New batteries come up to full capacity after about 4 to 5 charge discharge cycles.